The present invention relates to a radar of FM-CW system used for fast inter-vehicle distance measurement in highway traffic systems and, more particularly, to a radar capable of reducing sneak noise between the transmission and reception.
A prior art radar will now be described with reference to FIGS. 10 and 11. FIG. 10 is a block diagram showing a prior art FM-CW radar for inter-vehicle distance measurement. Referring to the Figure, the illustrated FM-CM radar comprises a transmitting unit 101, a circulator 102, a transmitting/receiving antenna 103, a receiving unit 104, and a signal processing unit 105 for judging a target by calculating the distance thereof, relative speed thereof and so forth.
The transmitting unit 101 includes a triangular modulating signal generator 101A for generating a triangular modulating signal, an oscillator 101B for generating a continuous oscillation signal by frequency modulating a carrier with the triangular modulating signal, and an amplifier 107D for amplifying the oscillation signal coupled from the oscillator 101B through a distributor 101C. The amplified oscillation signal from the amplifier 101D is coupled through the circulator 102 to the transmitting/receiving antenna 103 and transmitted toward a target (not shown).
The receiving unit 104 includes a mixer 104A for mixing a reflected wave signal from a target, which is received by the transmitting/receiving antenna 103 and the oscillation signal coupled from the oscillator 101B through the distributor 101C to extract a beat signal, and an amplifier 104B for amplifying the frequency band of the beat signal output of the mixer 104A.
The signal processing unit 105 includes an A/D converter 105A for converting the beat signal from the amplifier 104B in the receiving unit 104 to a digital signal, a frequency analyzer 105B for analyzing the digital beat signal from the A/D converter 105A to obtain a frequency characteristic and a spectral characteristic through fast Fourier transformation (FFT), and a microcomputer (CPU) 105C for computing the distance and azimuthal data of the target or relative speed thereof according to the results of analysis in the frequency analyzer 105B.
FIG. 11 is a block diagram showing a different prior art FM-CW radar. In the Figure, elements like those shown in FIG. 10 are designated by like reference numerals. This FM-CW radar comprises, in addition to a transmitting unit 101, a receiving unit 104 and a signal processing unit 105 having the same structures as those shown in FIG. 10, a transmitting antenna 106 and a receiving antenna 107. The antennas 106 and 107 are provided independently while omitting the circulator.
In this antenna, the oscillation signal from the transmitting unit 101 is thus transmitted directly from the transmitting antenna 106 toward a target. Also, the reflected wave from the target is received in the receiving unit 104 through the exclusive receiving antenna 107.
In the prior art FM-CW radar shown in FIG. 10, the noise components that have adverse effects on the reception sensitivity of the receiving unit 104 are those introduced as phase noise from the transmitting unit 101 through the circulator 102 to the receiving unit 104. To suppress such sneak noise, it is conceivable to reduce the phase noise in the oscillator 101B or increase the isolation of the circulator 102.
However, increasing the isolation of the circulator is difficult particularly for a high frequency of a millimeter wave. Even if it is possible, such problems as the necessity of a time-consuming circulator adjusting operation and cost increase are posed. Reducing the oscillator phase noise, on the other hand, poses such a problem as it is difficult to sufficiently suppress the phase noise in case when it is designed such as to provide a great oscillator frequency range.
With the prior art FM-CW radar shown in FIG. 11, having the separately provided transmitting and receiving antennas 106 and 107, interference between the transmission and reception can be prevented by the isolation between the two antennas. It is thus possible to solve the problems in the radar shown in FIG. 10.
In this radar, however, restriction is imposed on the disposition of the two, i.e., transmitting and receiving antennas. Particularly, when the beam number is increased to increase the azimuthal resolution, the radar housing size is increased.